This invention relates to an overlay cell type mobile communication system for performing radio communications by forming and laying a macro cell of a macro cell system and a plurality of micro cells of a micro cell system on each other and permitting the macro cell system and the micro cell system to commonly use part or all of a plurality of carrier frequencies.
In recent years, with an increase in the communication needs and the development of the communication technique, various types of mobile communication systems have been developed. Among them, an overlay cell type mobile communication system is included. The overlay cell type mobile communication system performs radio communications by forming and laying a macro cell with the cell radius of approx. several km, for example, and a plurality of micro cells with the cell radius of approx. several ten meters, for example, on each other on the same service area and causing the cells to use a common carrier frequency. In the above type of system, since the micro cell and the macro cell use the common carrier frequency, the utilization factor of the frequency can be enhanced in comparison with a system using different carrier frequencies.
FIG. 10 is a schematic construction view showing one example of an overlay cell type mobile communication system which is conventionally provided. In FIG. 10, Ea1, Ea2, . . . indicate macro cells and the macro cells Ea1, Ea2, . . . are formed by macro cell base stations BSa1, BSa2, . . . Further, Eb1, Eb2, Eb3, . . . indicate micro cells and the micro cells Eb1, Eb2, Eb3, . . . are formed by micro cell base stations BSb1, BSb2, BSb3, . . . . A mobile station MS of the macro cell system performs radio communications with the macro cell base stations BSa1, BSa2, . . . . A mobile station PS of the micro cell system performs radio communications with the micro cell base stations BSb1, BSb2, BSb3, . . . .
For example, a frequency division multiple access-frequency division duplex (which is hereinafter referred to as FDMA-FDD) scheme is used in the macro cell system and a time division multiple access-frequency division duplex (which is hereinafter referred to as TDMA-FDD) scheme is used in the micro cell system.
FIG. 11 shows one example of assignment of radio channels in the macro cell system and the micro cell system. Assume now that the mobile station MS of the macro cell system performs radio communication with the macro cell base station BSa1 in the macro cell Ea1 by use of an up channel CHUP1 using a carrier frequency F11 and a down channel CHDW1 using a carrier frequency F21. Further, assume that a call relating to the mobile station PS is issued in this condition in the micro cell Eb1 formed in the adjacent macro cell Ea2. Then, the micro cell base station BSb1 of the micro cell Eb1 searches for carrier frequencies which the micro cell system holds. Based on the result of the searching operation, the micro base station selects a carrier frequency which will not give any influence on the communication of the macro cell system and which satisfies the condition of the ratio (D/U) of a desired wave to an interference wave and then assigns the selected carrier frequency to the mobile station PS.
For example, one pair of up carrier frequency F11 (up channel CHaUP1) and down carrier frequency F21 (down channel CHaDW1) which are used by the macro cell Ea1 and are not used by the macro cell Ea2 are selected. Then, as shown in FIG. 11, an up channel CHbUP1 and a down channel CHbDW1 for micro cell are set on the channels CHaUP1 and CHaDW1 and the channels CHbUP1, CHbDW1 are assigned to the mobile station PS. After this, the mobile station PS starts radio communication with the micro cell base station BSb1 based on the FDD system by use of the pair of up channel CHbUP1 and down channel CHbDW1.
If a call relating to a different micro cell mobile station is issued at the same time, the micro cell base station BSb1 sets an up channel CHbUP2 and a down channel CHbDW2 for micro cell on the carrier frequencies F11, F21 used by the macro cell Ea1 as shown in FIG. 11, for example, and then assigns the thus set channels to the above different mobile station.
That is, in the conventional overlay cell type system, the up channel and down channel of the micro cell are assigned to the up carrier frequency and down carrier frequency for the macro cell so as to perform radio communication based on the FDD system.
However, the system which is conventionally proposed has the following problems. That is, the up carrier frequency and down carrier frequency used for constituting one bi-directional radio channel in the macro cell system are different in the transmission station and radio signal propagation path. For this reason, the degrees of interference given by the radio signals of the up channel and down channel to the micro cell are different from each other in some position of the mobile station MS for macro cell as shown in FIG. 10. Therefore, in the conventional overlay cell type system, if both of the up carrier frequency and down carrier frequency used for constituting one bi-directional radio channel in the macro cell system do not satisfy a preset condition such as the ratio D/U described before, it becomes impossible to select the above carrier frequencies and assign them to the mobile station of the micro cell system.
Therefore, it takes a long time to detect one pair of carrier frequencies which satisfy a preset condition required for the carrier frequencies to be assigned to the mobile station of the micro cell system. The number of pairs of carrier frequencies which can be assigned is small and it is difficult to obtain a sufficiently large number of channels required by the micro cell system.
Further, even when the up and down channels which satisfy the above condition are assigned and the communication quality of one of the carrier frequencies is kept high, it becomes impossible to maintain preferable communication if the interference in the other carrier frequency increases during the communication. As a result, the communication quality tends to be deteriorated during the communication and the probability that one communication process can be smoothly completed, that is, the rate of successful completion of communication becomes low.